The production of phenol and/or cyclohexanone from cyclohexylbenzene (CHB) is an emerging technology, interesting in that it co-produces cyclohexanone, rather than acetone. CHB may be produced, for example, by direct alkylation of benzene with cyclohexene, or as disclosed in U.S. Pat. No. 6,037,513, by contacting benzene with hydrogen in the presence of a catalyst. The cyclohexylbenzene may then be oxidized to the corresponding hydroperoxide and the hydroperoxide cleaved to phenol and cyclohexanone using a catalyst. Depending upon need or demand, the phenol and cyclohexanone may each be taken as products, and/or the phenol can be hydrogenated to produce additional cyclohexanone, and/or the cyclohexanone can be dehydrogenated to produce additional phenol. Cyclohexanone is widely used to make caprolactam, which, in turn, is used for making nylon-6, a widely used polymer material. Phenol may be used to make a wide variety of chemical products, including bis-phenol A, polycarbontaes, phenolic resins, and the like.
In such processes for the co-production of phenol and cyclohexanone, the oxidation of CHB to cyclohexylbenzene-hydroperoxide (CHB-HP) may be a gas-liquid oxidation that takes place through a free radical chain reaction homogeneously catalyzed by an N-hydroxy-substituted cyclic imide, such as N-hydroxyphthalimide (NHPI), for instance as described in WO 2014/137623. In particular, the liquid-phase reaction medium comprising the CHB is contacted with an oxygen-containing gas (e.g., air or O2) to form the CHB-HP. Desirably, the oxidation is conducted at low per-pass conversion to optimize process yields, because the desired CHB-HP product is a reaction intermediate. One way to achieve this result is through a shorter residence time of the reactants, ensuring that only a small portion of the gas-phase oxygen reacts with the liquid-phase CHB. However, such a process entails a large degree of oxygen bypass (e.g., un-used oxygen passing through the liquid-phase reaction medium). This problem can be compounded because the kinetics of the gas-liquid oxidation of CHB to CHB-HP are relatively slow, which means that highly concentrated zones of oxygen-containing gas within the liquid-phase reaction medium may form and pass unreacted through the liquid-phase reaction medium, only to collect in the vapor-phase overhead in the reactor. Such excessive oxygen bypass will lead to quickly rising oxygen concentration in the headspace, which will result in dangerous flammable conditions if not controlled. Therefore, a sufficient amount of oxygen must be supplied to the liquid phase reaction medium in order to maintain the desired conversion rate, while at the same time, delivery of the oxygen-containing gas must be carried out so as to maintain the oxygen concentration in the vapor phase headspace of the reactor below the flammability limit for safe operation.
One option is a lower gas flow rate into the reactor. However, this reduces the speed at which cycloalkylaromatic feed is converted to the desired cycloalkylaromatic hydroperoxide, which negatively impacts the process economics. Therefore, a better solution is desired.
The present inventors have discovered a means to maintain the desired per pass conversion and speed of conversion, while minimizing the amount of oxygen that will be present in the reactor headspace, thereby allowing for safe operation of the oxidation reactor below the overhead oxygen concentration flammability limit. In particular, a gas distributor of the particular designs according to various embodiments described herein maximizes the distribution of vapor phase oxygen-containing gas through the liquid phase reaction medium, thereby maximizing efficiency of the reaction and minimizing gas bypass through the liquid phase reaction medium and into the reactor headspace. In addition, liquid distributors instead of or in addition to such gas distributors in yet further embodiments of the invention described herein may further enhance the mixing of reactants through the liquid phase reaction medium.
Further, although described in the context of CHB oxidation to CHB-HP, it is believed that the presently disclosed invention may be equally applicable to the oxidation of any liquid-phase organic reactant using an oxygen-containing gas, particularly oxidation reactions featuring relatively slow reaction kinetics and requiring carefully controlled per-pass conversion.
Although oxidation of CHB to CHB-HP via N-hydroxy-substituted cyclic imides has been disclosed, the inventors are not aware of any prior published recognition of the problem of gas bypass (let alone a solution to such problem) in such a reaction on an industrial scale, nor are the inventors aware of any current commercial practice of such oxidation reactions on an industrial scale. Some references of interest may include: Kulkarni, A. V. & Joshi, J. B., Design and selection of sparger for bubble column reactor. Part I: Performance of different spargers, CHEM ENG'G RESEARCH & DESIGN 89 (2011), 1972-1985; Kulkarni, A. V., Joshi J. B., Design and selection of sparger for bubble column reactor. Part II: Optimum sparger type and design, CHEM ENG'G RESEARCH & DESIGN 89 (2011): 1986-1995; US Patent Application Publication Nos. 2014/0148569, 2014/0336417, 2014/0371490; and WIPO patent publication WO 2014/137623.